A swing arm for a motorcycle is configured so that the front end thereof is rotatably connected to a chassis framework via a rear suspension and the rear end thereof rotatably supports a rear wheel (see Japanese Patent Application Publication No. 2012-076612). The swing arm disclosed in Japanese Patent Application Publication No. 2012-076612 includes a base section which is connected to the chassis framework via a pivot shaft, and a pair of right and left arm sections which extend rearwards from the base section so that the rear wheel is disposed between the arm sections. The arm sections rotatably support the axle of the rear wheel at the rear ends thereof. The swing arm is configured to absorb a motion of the chassis framework in a vertical direction.
In the swing arm disclosed in Japanese Patent Application Publication No. 2012-076612, inner surfaces of portions of the respective arm sections including rear ends of the respective arm sections are formed in parallel with the center line of the motorcycle and inner corner parts bridging between the arm sections and the base section are formed into circular arc shapes as viewed from the top. In order to secure a clearance between the rear when and the respective arm sections, the dimension of the radius of the circular arc have to be small. Accordingly, the corner parts are likely to be subjected to a large stress concentration, thereby causing disadvantage in the rigidity. This problem becomes remarkable when the width of the respective arm sections in the with direction of the motorcycle is made thin in order to reduce the rigidity of the arm sections against an external load in the width direction.
By the way, the motorcycle equipped with such a swing arm is able to maintain a very deep banking angle upon turning, due to an improvement in technology such as in a field of tires. This is remarkable in racing or super-sports motorcycles.
As a suspension can absorb the motion of the chassis framework only in the vertical direction, in the motorcycle capable of maintaining the deep baking angle upon turning, there is a growing need for, in addition to an improvement in torsional rigidity for improving straight-motion stability or maneuverability, the optimization of lateral bending rigidity related to the absorbency or stability upon turning, by absorbing a lateral force applied from the irregular road to the chassis framework with the deformation of the chassis framework or the swing arm.
The swing arm for the motorcycle disclosed in Japanese Patent Application Publication No. 2012-076612, however, less considers the lateral bending rigidity, but merely employs a structure in which a front side base section of the swing arm is strengthened and arm sections holding an axle is connected thereto in order to efficiently realize improved torsion rigidity and weight saving.
In order to reduce the lateral bending rigidity in such a motorcycle which adopts such a structure, is it necessary to employ a structure in which the arm sections for holding the axle of the real wheel is made thin so as to be greatly deformed. In this case, however, the stress concentration occurs on the connection parts between the base section and the respective arm sections when a lateral force is applied to the swing arm. Because of this, the connection parts are required to be reinforced, causing a problem of increase in weight of the chassis framework. Another problem also arises in that, even with the reinforced connection parts, the lateral bending rigidity may not essentially be reduced to a desired value.